This invention relates to a speed indicator for hang gliders or the like having an audio output whose frequency varies in proportion to air speed.
U.S. Pat. No. 2,386,992 to Trott discloses an audible stall speed indicator for aircraft wherein Pitot tubes fitted with venturi sections are coupled to variable capacitor pressure sensors of the diaphragm type. The capacitors control oscillator frequencies whose outputs beat against an audio oscillator output to produce a composite output signal that peaks in both frequency and amplitude at the stall speed. The relative complexity and excessive hardware requirements of the Trott device render it unacceptable for use with small, inexpensive, lightweight airborne vehicles, such as hang gliders.
U.S. Pat. No. 914,959 to Lallie discloses a liquid flow meter having an impeller driving an electrical make-and-break switch, or circuit interrupter, series connected in an audio pulsing circuit. The device is intended to enable flow measurement by having a listener/operator simply count the number of generated pulses. The Lallie device would not appear to be operable, even with major modifications, in a relatively slow speed airborne vehicle such as a hang glider.
Other prior art references developed during the course of a patentability search on this invention, but whose teachings do not appear sufficiently pertinent or germane to warrant detailed discussion, are as follows:
U.s. pat. No. 1,267,633 -- F. H. Clift PA1 U.s. pat. No. 2,311,395 -- A. A. Johnson PA1 U.s. pat. No. 2,706,406 -- R. P. Vincent et al. PA1 U.s. pat. No. 3,610,039 -- J. Althouse et al. PA1 U.s. pat. No. 3,739,367 -- G. H. Fathauer PA1 U.s. pat. No. 3,859,629 -- K. Komiyama et al. PA1 U.s. pat. No. 3,919,688 -- G. Schick